Weeds Hitch a Ride with Container-Grown Ornamentals

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Container-grown ornamentals are a staple of the nursery industry and give homeowners a quick way to incorporate established plants into backyard gardens and landscapes. But without proper management, they also can serve as a superhighway for the spread of weeds. The Weed Science Society of America urges both homeowners and growers to take appropriate precautions.

Growers and retailers who exercise good weed control practices can halt the spread of weeds through container-grown ornamentals. Nursery photo courtesy of Syngenta.

Prevention is always the most powerful and cost-effective approach for combating weeds

Container-grown ornamentals are a staple of the nursery industry and give homeowners a quick way to incorporate established plants into backyard gardens and landscapes. But without proper management, they also can serve as a superhighway for the spread of weeds.

In Alaska where the number of nonnative invasive plants and weeds has surged in recent years, researchers with the U.S. Department of Agriculture's Agricultural Research Service* recently took a close look at the problem and developed some interesting insights.

"The team examined container-grown plants to see if they were a contributing factor," said Lee Van Wychen, science policy director for the Weed Science Society of America. "The research showed significant numbers of nonnative weeds species were hitchhiking across the state in the same containers as ornamental flowers and shrubs."

The two-year study examined a variety of container-grown plants, including vegetables and herbs, the showy perennials prized by backyard gardeners, and the woody plants, trees and shrubs that are a landscaping mainstay for both homes and businesses. More than two dozen plants were purchased from each of 29 nurseries selling to the Alaska market - including four from within the state and 25 from the western U.S. and Canada.

After incubating the soil from each container in a greenhouse to see what sprouted, researchers found 54 weeds or invasive plants had been transported alongside the container-grown ornamentals. The five most common included: sticky chickweed (Cerastium glomeratum Thuill.); hairy bittercress (Cardamine hirsuta L.); common groundsel (Senecio vulgaris L.); La Plata sandspurry (Spergularia plantensis [Cab.] Fenzl); and birch (Betula sp.). Researchers also discovered Canada thistle (Cirsium arvense L.), a species prohibited in Alaska by law. Of the 54 plants that sprouted, only three were native to Alaska.

"Based on the Alaska data, it is clear that container plants play a role in the spread of weeds that can be a risk to native plants and wildlife habitats," Van Wychen said.    

Researchers found that several variables affected the number of weeds found:

  • Soil from balled or burlap-covered trees and shrubs held more weed seeds than vegetable, herb and perennial containers.
  • More weeds sprouted from soil-based mixes and mineral soil than from sterile, soilless potting mixes like sphagnum peat moss.
  • The grower or vendor made a difference. It was evident that several had superior weed control practices and sold plants with few if any weeds.

"Prevention is always the most powerful and cost-effective approach for combating weeds," Van Wychen said. "We urge growers and retailers to make weed control a routine part of their operations and urge homeowners to be vigilant. If you spot weeds that sprout alongside your new container-grown plants, pull them before they produce seed and spread. It can make a real difference."

About the Weed Science Society of America:
The Weed Science Society of America, a nonprofit professional society, was founded in 1956 to encourage and promote the development of knowledge concerning weeds and their impact on the environment. The Weed Science Society of America promotes research, education and extension outreach activities related to weeds, provides science-based information to the public and policy makers, and fosters awareness of weeds and their impacts on managed and natural ecosystems. For more information, visit http://www.wssa.net.

*Characterizing Pathways of Invasive Plant Spread to Alaska: I. Propagules from Container-Grown Ornamentals. Jeffrey S. Conn, Cassie A. Stockdale and Jenny C. Morgan. Invasive Plant Science and Management 2008. 1:331-336.

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